[ci skip] Add link reference in ActiveSupport::PerThreadRegistry

[alexcameron89]

Module#thread_mattr_accessor is referenced in ActiveSupport::PerThreadRegistry documentation, and the link makes it easier to find the documentation for the method and friends.

[rails-bot]

r? @sgrif

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[maclover7]

Hmm, is this working for you locally? When I run rdoc locally and pull up the PerThreadRegistry documentation page, this rdoc-ref doesn't work for me :/

[alexcameron89]

I checked it with bundle exec rake rdoc and it works. Can you try it with that?

[maclover7]

👍 works for me now. Just realized I had been running rdoc off of master, not off of your branch -- sorry for the troubles. :)

👍 Thank you for your contribution!

❤️ 💚 💙 💛 💜

[alexcameron89]

Thank you! Y'all are awesome. 😄